The present invention relates to a multiple bore marine riser with a flexible reinforcement. Marine risers are applied in marine oilfield operations to provide a fluid communication between a well or a pipeline situated on or near the sea or ocean obttom, and a structure that is floating at the water level above the well or pipeline. A multiple bore marine riser consists of an assembly of parallel flowlines co-operating with a base member. The top of the marine riser is tensioned in vertical direction to obviate buckling of the flowlines and to reduce bending stresses in the flowlines which stresses result from the combined action of the waves, the water current and the displacement of the floating structure from the position thereof vertically above the base member. The flowlines may be used for various purposes, such as for the transport of fluids (such as oil or gas) from a well (or pipeline) to the floating structure, for the injection of fluids (such as gas or water) from the floating structure into a submerged well or plurality of submerged wells, for pumping fluids from the floating structure to a loading buoy or to shore, for carrying out "through-the-flowline" (TFL)-operations, wherein equipment is pumped to the well (and retrieved therefrom) by means of TFL-techniques, for flushing liquids through flowlines before disconnecting the multibore marine riser, and for supplying pressure fluids to submerged control equipment for operation submerged valves, couplings, etc. on the wellhead.
The present invention relates in particular to a multibore marine production riser with flexible reinforcement, wherein the flowlines are made of metal, such as steel. The flexible reinforcement is suitable for application at locations of the multibore marine riser where this riser is exposed from time to time or continuously to large bending forces. One such location is at the lower end of the multibore marine riser where the riser is connected to a base member that is anchored to the sea bottom, as shown in U.S. Pat. No. 3,605,413. Also, large bending forces can be expected to be exerted on the marine riser at the location where the latter may incidentally come into contact with the floating structure that supports the upper end of the marine riser, as shown in U.S. Pat. No. 3,602,319. The flexible reinforcement according to the invention may also find useful application at these locations.
It will be appreciated that damage of the marine riser by such large bending forces should be prevented in order to lengthen the operational life of the marine riser.
In a prior art construction designed for reducing bending moments in a multiple bore marine riser, each flowline is connected to fluid communication means on the base member by means of an elastomeric flexjoint. A drawback, however, is that angles of significant value between the parts of each flexjoint will prohibit the use of tools that are transported through the flowlines into the well and vice-versa, such as is done in "through-the-flowline" (TFL) operations.
In another prior art construction, the lower ends of the assembly of flowlines are rigidly connected substantially perpendicularly to the base member. The flowlines are guided through guide rings that are carried by a centrally arranged conduit of larger cross-section than the flowlines, which conduit is coupled to the base member by means of a universal elastomeric flexjoint. At lateral displacements of the conduit, the main axial load is taken up by this conduit and the guide rings carried thereby curve the individual flowlines according to a large bending radius as a result whereof relatively low bending stresses will be induced in the flowlines. The axial loads in the flowlines are relatively small, but sufficient to prevent buckling of the flowlines. The large bending radii of the flowlines allow the application of all TFL-operations. However, in a number of applications of a multiple bore marine riser, there is no use for the large-diameter flowline that is applied in this design for taking up the axial load. The presence of the load-carrying conduit is not present and the flowlines share the axial load exerted on the riser assembly.
Another object of the invention is a multiple bore marine riser provided with reinforcement means that protect the riser from being damaged by the floating structure from which it is suspended, when this structure is subjected to large displacements from the position thereof vertical above the base member to which the lower end of the riser is connected.